1. Field of the Invention
The present invention relates to a magnetic random access memory for magnetically storing information by utilizing a magnetic storage element. More specifically, the present invention relates to a structure of the magnetic random access memory, a method of accessing the magnetic random access memory and a method of manufacturing the magnetic random access memory.
2. Description of the Related Art
In recent years, a magnetic random access memory (MRAM) has vigorously been studied. The magnetic random access memory implies a nonvolatile memory for recording data by utilizing the magnetism of a magnetic storage element. FIG. 1A is a diagram of a memory cell 150 of the magnetic random access memory. The memory cell 150 is constituted by one magnetic storage element 151 and a metal oxide film semiconductor field effect transistor (MOSFET) 152. A writing word line 154 for writing information to the magnetic storage element 151 is provided in the vicinity of the magnetic storage element 151. The transistor 152 has a first electrode 152-1 connected to a reading word line 155 for reading information from the magnetic storage element 151 and has a second electrode 152-2 connected to the magnetic storage element 151. A third electrode 152-3 of transistor 152 is grounded. Moreover, a bit line 153 is connected to the magnetic storage element 151. The bit line 153 is utilized for writing information to the magnetic storage element 151 and reading the information from the magnetic storage element 151. According to the above description, apparently, the memory cell 150 has such a structure that one magnetic storage element 151 is present for one reading word line 155 and one transistor 152.
FIG. 1B is a sectional view of the memory cell 150. As shown, the magnetic storage element 151, the bit line 153 and the writing word line 154 are stacked on the reading word line 155 or the transistor 152. The magnetic storage element 151 is smaller than the transistor 152. Therefore, the size of the memory cell 150 is determined by the size of the transistor 152. Accordingly, if the size of the transistor 152 is reduced, a cell area is decreased correspondingly so that the degree of integration can be enhanced.
However, an enhancement in the degree of integration based on a reduction in the size of the transistor 152 also has limitations. In more detail, it is necessary to determine the resistance of the transistor 152 corresponding to the characteristic of a film depending on a GMR element or a TMR element to be used as the magnetic storage element 151. However, the resistance of the transistor 152 also depends on a dimension thereof (condition 1). Furthermore, one transistor 152 is always present in one memory cell 150 (condition 2). Accordingly, the size and number of the transistors 152 which can be integrated into a chip having a constant area are considerably restricted according to the conditions 1 and 2. Therefore, it is impossible to obtain a memory having a small area, the small number of elements and a large capacity.
It is an object of the present invention to provide a magnetic random access memory in which the degree of integration of a memory is enhanced and the number of necessary components is small through the provision of a plurality of magnetic storage elements on a single transistor.
It is another object of the present invention to provide a method of writing information to the magnetic random access memory and a method of manufacturing the magnetic random access memory.
The present invention provides a magnetic random access memory comprising a plurality of magnetic storage elements for magnetically storing information, respectively, a plurality of bit lines connected to the magnetic storage elements, respectively, a plurality of writing word lines provided corresponding to the vicinity of the magnetic storage elements and utilized for applying magnetic field to write the information, respectively, and a switch including a first terminal and a second terminal, the first terminal being connected to a single reading word line to be utilized for determining whether current is caused to flow to the second terminal and the second terminal being connected to each of the magnetic storage elements. Consequently, the above-mentioned object can be achieved.
Preferably, the switch further include a third terminal which is different from both the first terminal and the second terminal and to which a predetermined voltage is applied, current flowing to the second terminal through the third terminal based on the voltage applied to the first terminal.
Preferably, the magnetic storage elements are laminated in a vertical direction to the switch.
Preferably, the writing word line is orthogonal to the bit line and the magnetic storage element is positioned between the writing word line and the bit line.
The present invention provides a method of writing information to a magnetic random access memory, comprising the steps of causing bit line writing current to flow to an object bit line corresponding to an object magnetic storage element, causing word line writing current to flow to an object writing word line corresponding to the object magnetic storage element, and changing a direction of magnetization of the object magnetic storage element into a direction corresponding to the information through the magnetic field generated by the bit line writing current and the word line writing current. Consequently, the above-mentioned object can be achieved.
Preferably, the method of writing information further comprises the step of causing current to flow to at least one bit line other than the object bit line.
Preferably, the current to flow to the at least one bit line has a smaller magnitude than that of the bit line writing current.
Preferably, at the step of causing current to flow to the at least one bit line, a direction of current flow is changed corresponding to a positional relationship between the at least one bit line and the object bit line.
Preferably, at the step of causing current to flow to the at least one bit line, the current is caused to flow in the same direction as the object bit line if the at least one bit line is positioned above the object bit line, and the current is caused to flow in a reverse direction to the object bit line if the at least one bit line is positioned below the object bit line.
Preferably, the information is recorded to represent an N-digit binary number by using the N magnetic storage elements.
The present invention provides a method of reading information from an object magnetic storage element, comprising the steps of applying a reading word line voltage to a single reading word line, causing bit line reading current to flow to an object bit line corresponding to the object magnetic storage element, detecting a terminal voltage of the object magnetic storage element, and deciding contents of the stored information based on the terminal voltage. Consequently, the above-mentioned object can be achieved.
Preferably, the method of reading information from the magnetic random access memory further comprises the steps of causing reference current to flow to a reference bit line different from the object bit line, detecting a reference terminal voltage of the magnetic storage element connected to the reference bit line, and deciding contents of the stored information based on the reference terminal voltage and the terminal voltage.
The present invention provides a method of manufacturing a magnetic random access memory, comprising the steps of (a) providing a substrate, (b) forming a single transistor including a first electrode, a second electrode and a third electrode on the substrate, the second electrode being conducted to the third electrode corresponding to a voltage to be applied to the first electrode, (c) forming a writing word line insulated from the single transistor, (d) forming a magnetic storage element connected to the second electrode of the single transistor for storing information in the vicinity of the writing word line, (e) forming a bit line connected to the magnetic storage element and insulated from the writing word line, and (f) repeating the steps (c) to (e). The above-mentioned object can be achieved.
For example, at the steps (c) to (f), the writing word line, the magnetic storage element and the bit line are laminated in a vertical direction to the single transistor formed on the substrate.
For example, the step (d) further includes the step of forming a single reading word line to be connected to the first electrode.
Preferably, at the steps (c) to (e), the writing word line is orthogonal to the bit line, and the writing word line, the magnetic storage element and the bit line are formed such that the magnetic storage element is positioned between the writing word line and the bit line.
Preferably, the step (b) further includes the step of giving a predetermined voltage to the third electrode.
According to the magnetic random access memory of the present invention, a plurality of magnetic storage elements are connected to a single transistor. Therefore, it is possible to obtain a memory having the small number of components and high degree of integration. More specifically, the magnetic storage elements are laminated in the vertical direction to the transistor. Therefore, the degree of integration can be enhanced dramatically.